Causality in the relativistic bound-state problem

Abstract

Although the exact Bethe-Salpeter equation is certainly the appropriate field-theoretic framework to describe the non-perturbative problem of scattering and bound states, the inevitable truncations introduce inconsistencies such as loss of symmetries or incorrect one-body limit. I conjecture that these problem can be overcome if the truncation preserves the field-redefinition invariance of the exact equation. A sum rule for light-by-light scattering can provide a testing ground of this conjecture.Comment: 4 pp, 2 figs; Contribution to the 5th Asia-Pacific Conference on Few-Body Problems in Physics (APFB11